1. Field of the Invention
This invention relates to chemical mechanical polishing. More specifically, the present invention provides a polishing head for a chemical mechanical polishing apparatus. Moreover, the present invention provides a method for polishing a substrate by employing the polishing head of the present invention.
2. Description of the Prior Art
Sub-micron integrated circuit devices are formed on substrates such as semiconductor wafers by patterning conductive or interconnect film layers (e.g., aluminum (Al), titanium nitride (TiN), etc.) which have been deposited on nonconductive or intermediate dielectric film layers (e.g., silicon oxide (SiO.sub.x)). In order to pattern or etch the interconnect film layer, the exposed surface of the interconnect film layer must be topographically planar. An intermediate dielectric film layer having a non-planar surface will transfer its topographical profile to that of the deposited interconnect film layer. As a result, prior to the deposition of the interconnect film layer, the surface of the intermediate dielectric layer has to be planarized. To pose the problem more concretely, the patterning and etching step is prepared by selectively developing photoresist layers on the exposed surface of the interconnect film layer. A non-planar surface prevents the focusing of a photolithography apparatus on the entire exposed surface of the interconnect film layer for the exposure of the photoresists. As a result, the interconnect film layer having a surface defined by a non-planar topography cannot be etched or patterned by photolithographic techniques. The syllogism follows that the intermediate dielectric film layer, on which the interconnect film layer is deposited, must have a planarized surface.
Chemical mechanical polishing (CMP) is one recognized method of planarization. CMP technique requires that the substrate be mounted on a polishing head with the surface of the substrate to be polished exposed. The polishing head, supporting the substrate, is then placed against a linear polishing belt or a planar polishing pad. Referring to FIGS. 32A and 32B, which are schematic side elevational and front elevational views of a linear CMP apparatus, generally illustrated as 300, there is seen a continuous, vertical polishing belt 302 configured to polish a vertically held substrate, such as a semiconductor wafer 82. A polishing head 10 positions the substrate 82 against a polishing pad (omitted from the Figures), which is attached to the vertical polishing belt 302. The polishing belt 302 is kept in continuous motion, as indicated by arrow 308, by rotating pulleys 310 and 312 at a selected polishing speed (e.g., 1-1000 ft/min). A support head 314 provides a base for the application of pressure (e.g., 1-10 psi) by the polishing head 10 against the substrate 82. The polishing head 10 may rotate in a clockwise or counter-clockwise direction, as indicated by arrow 316, and may oscillate or translate back and forth, as indicated by arrow 320, by an oscillating arm 318 of a driving mechanism (omitted from the Figures). Moreover, a slurry, typically a mixture of an abrasive and at least one chemically reactive agent, is supplied to the polishing pad. Accordingly, a chemical reaction and a mechanical abrasion is provided at an interface between the substrate 82 and the polishing pad.
A planar CMP apparatus 400, as illustrated in FIGS. 33A and 33D includes the polishing head 10, horizontally supporting the substrate 82. The polishing head 10, as mentioned above, rotates in a clockwise or counterclockwise direction, as indicated by the arrow 316, and is oscillated back and forth, as indicated by the arrow 320, by the oscillating arm 318 of the driving mechanism (omitted from the Figures). However, in lieu of the continuous, vertical polishing belt 302, a rotating, planar polishing platen 402 is provided. The planar polishing platen 402 supports and rotates a polishing pad 404 about a driving shaft 406. The rotation of the polishing platen 402 is indicated by arrow 408. A slurry is provided to the polishing pad 404 for providing the abrasive chemical solution.
A recurring problem with the CMP process is the inconsistency in the removal rates across the surface of a substrate. CMP has a tendency to differentially polish, i.e., over-polish or under-polish, particular localized regions of the substrate surface. For example, an outer circumferential region, typically 5-20 mm in length, is an area where over-polishing at times occurs. As a result, the substrate procures a non-planar, convex shaped surface, impeding any further use of photolithography for patterning purposes. Therefore, the CMP apparatus should have a polishing head which increases surface planarity and polishing uniformity.